Methods for the copolymerization of epoxides and carbon dioxide to form aliphatic polycarbonates (APCs) have been known in the art since the 1960s. The early methods of copolymerization used catalysts based on heterogeneous zinc compounds and suffered from low reactivity, a lack of selectivity for polymer formation vs. cyclic carbonate formation, and a tendency to produce polycarbonates contaminated with ether linkages.
Improved methods using catalysts based on transition metals have been discovered over the past decade or so. These newer catalysts have increased reactivity and improved selectivity. Nevertheless, even using highly active catalysts such as those disclosed in U.S. Pat. No. 7,304,172, the reaction times required to make high molecular weight polymer are typically quite tong. In addition, the best-performing catalysts disclosed in the '172 patent require the addition of a separate co-catalyst to achieve optimum activity.
Recent advances in addressing these shortcomings have been made by developing transition metal catalysts with one or more functional groups tethered to a ligand of the catalysts, see for example WO 2010/022338, WO 2008/136591, and WO 2010/013948 and references cited therein. However, these catalysts with tethered functional groups suffer from undesirable induction times prior to onset of polymerization, see for example (Angew. Chem. Int. Ed. 2008, 47, 7306-7309. In some cases, the induction time prior to the onset of polymerization can exceed the actual reaction time. This is inefficient, causes unpredictability in the total processing time required to make polymer, and reduces the ultimate catalyst productivity that can be achieved. This is highly undesirable in a manufacturing context. As such, there remains a need for methods that will reduce or eliminate the induction time in epoxide CO2 copolymerization reactions.